wrote

You failed maths at school, didn't you?

I know this can be confusing but it is really very easy to understand,
and quite basic knowledge too. I try to make this as easy as possible:

Say you take a picture using 2 second exposure time. Each pixel will
collect some amount of photons depending on the luminance of the
corresponding scene area. Now you take another picture with -1 stop
exposure adjustment e.g. you change the exposure time to 1 second. So
in this second picture you have collected just half the amount of
photons into each of the pixels compared to the first picture. Thus,
the effect you have made is the same as if you multiplied the captured
data of the first picture by 0.5 pixel by pixel.

Timo Autiokari